High speed switching driver

ABSTRACT

The present invention is generally directed towards a system and method for providing a high speed switching driver to transmit data information. A first current is switched through a diode or shunted through an alternate path to selectively bias the diode. A second current can also be selectively switched through the diode to modulate a light energy output transmitted from the diode. Based on this configuration, fast bias-switching for generating a modulated laser diode output can be achieved by selectively switching the bias current through the diode or alternate path. Consequently, a solid state device can be controlled to transmit data information after a relatively minimal setup time delay to bias the diode.

BACKGROUND OF THE INVENTION

[0001] Laser diodes are typically used to transmit data information overfiber optic networks. To achieve higher speed data rates, a laser diodecan be biased with a drive current so it is ‘ON’ and produces at least aminimal optical output. While the diode is biased, the diode can bedriven with additional current so that the light output of the diodevaries over time between two power output levels. One power output levelof the diode can represent a logic low while another power output levelof the diode can represent a logic high.

SUMMARY OF THE INVENTION

[0002] One aspect of the present invention is directed towards a systemand method for providing high speed switching to transmit datainformation. This can be achieved by switching current through a diodeor alternate path rather than adjusting a setting of a current sourcebetween 0.0 mA (to turn off the diode) and I_(BIAS) (to bias the diode).Complex circuitry often used in precision current sources ordinarily donot lend themselves to high speed adjustments that would be necessary torapidly shut off, bias or modulate a diode for transmitting.

[0003] In an illustrative embodiment, a current is switched through adiode or shunted through an alternate path to selectively bias a diodefor transmitting. When the current is switched through the diode, thediode can be biased for generating a light output. A second current canbe selectively switched through the diode to modulate a light energyoutput transmitted from the diode. Based on this configuration, fastbias-switching for generating a modulated laser diode output can beachieved by selectively switching the bias current through the diode oralternate path. Consequently, a solid state device can be controlled totransmit data information after a relatively minimal setup time delay tobias the diode.

[0004] A light output of the diode can be coupled to a fiber to transmitinformation over an optical network such as a passive optical network orcable television (CATV). In such applications, hi-speed biasing of thediode or multiple diodes can be used to more efficiently transmitinformation via time division multiplexing. That is, one of multiplediodes can be selectively activated in minimal time to transmit amodulated signal over a fiber network shared by multiple transmitters.The other diodes not transmitting can be almost completely shut down sothat they do not interfere with the activated diode.

[0005] A resistor can be disposed in series with the diode to dissipateenergy and reduce chirping of the circuit during the “switch over”process of biasing the diode. In one application, the resistor isgreater than 20 ohms so that the power output of the diode makes asmooth transition when it is switched or modulated. This resistor can besplit into two separate resistors, one in each of the modulation andbias current paths.

[0006] A current switched to bias or modulate the diode also can beadjusted to compensate for aging of the diode. For example, when a lightenergy output of the diode decays due to aging or the system changestemperature, the bias or modulation current can be increased ordecreased so that the diode still generates a bias and modulation outputat particular power levels.

[0007] One method for selectively switching the bias current involvesdisposing a first switch in series with the diode and a second switchalong an alternate path so that the bias current can be switched eitherthrough the diode or the alternate path depending on which of the twoswitches is activated. In such an application, switching can becontrolled so that one switch has a lower impedance than the other.Accordingly, the current can be directed through the switch (and path)having the lower impedance.

[0008] In one application, the first and second switches are driven witha differential signal to selectively bias the diode. More specifically,part of the differential signal can be used to control one switch whileanother part of the differential signal can be used to control the otherswitch. When the differential signal is binary, one of the pair ofswitches is typically active except during a transition period betweenchanging states. This transition period can be minimal when hi-speeddifferential drivers are used to control a state of the switches. Inthis instance, the bias current is generally either selectively switchedto bias the diode or is shunted through an alternate path. To providefast switching times, bipolar transistor switches can be driven with adifferential signal based on ECL (Emitter Coupled Logic) or othersuitable voltage levels.

[0009] In a similar manner as discussed for the bias current, themodulation current can be switched through the diode or anotheralternate path using a pair of switches such as bipolar transistorsdriven by a differential voltage signal to modulate the diode fortransmitting.

[0010] To ensure that the bias current is set properly, a power level oflight transmitted by the diode can be measured while the diode isbiased. Based on a measured output of the diode, feedback can be used toadjust the bias current so that the diode produces a light output withina specified range when biased.

[0011] The diode also can be calibrated for storing setting informationin memory. Typically, a bias and modulation current are selected so thata receiver of the optical signal generated by the diode can detectcorresponding logic levels of each data bit transmitted in a packet ortime frame.

[0012] The previously discussed aspects of the present invention haveadvantages over the prior art. For example, one application of thepresent invention involves transmitting data information from one ofmultiple laser diodes to a target device in a shared network. Since thelaser diode can be biased to transmit data information in minimal time,the bandwidth of the shared network can be increased.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013] The foregoing and other objects, features and advantages of theinvention will be apparent from the following more particulardescription of preferred embodiments of the invention, as illustrated inthe accompanying drawings in which like reference characters refer tothe same parts throughout the different views. The drawings are notnecessarily to scale, emphasis instead being placed upon illustratingthe principles of the invention.

[0014]FIG. 1 is a graph of input current versus light power output of alaser diode device according to certain principles of the presentinvention.

[0015]FIG. 2 is a block diagram of a burst mode laser system accordingto certain principles of the present invention.

[0016]FIG. 3 is a detailed schematic diagram of a burst mode lasersystem according to certain principles of the present invention.

[0017]FIG. 4 is a timing diagram illustrating a method to transmit fromthe burst mode laser system according to certain principles of thepresent invention.

[0018]FIG. 5 is a block diagram of multiple burst mode laser systems forcommunicating over a shared fiber network according to certainprinciples of the present invention.

[0019]FIG. 6 is a graph of light power output from each of multipleburst mode lasers transmitting at different time intervals according tocertain principles of the present invention.

[0020]FIG. 7 is a diagram of a network system according to certainprinciples of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0021] A description of preferred embodiments of the invention follows.

[0022]FIG. 1 is a graph showing the relationship between optical Powerand Current (P-I) for a laser diode. As shown, the power output (P) fromthe laser diode depends on the input pump current (I). As the input pumpcurrent (I) reaches a lasing threshold current I_(th,) the power outputP increases rapidly with each successive increase in input pump currentI.

[0023]FIG. 2 is a block diagram of a burst mode laser driver accordingto certain principles of the present invention. As shown, control system410 generates data signal 412 and bias signal 414 that respectivelydrive modulation current control circuit 545 and bias current controlcircuit 595. A combination of these circuits can be used to control thelight output generated by a solid state device such as diode 405. Thelight output generated by diode 405 can be coupled to fiber 430 of ashared medium network to transmit binary data information.

[0024] Control system 410 can include software, processor devices,memory devices and related circuitry to generate control signals 412,414 for selectively switching diode 405.

[0025]FIG. 3 is a detailed schematic diagram of a burst mode lasersystem according to certain principles of the present invention.Generally, circuit 545 controls a modulation current that is switchedthrough diode 405 while circuit 595 controls a bias current switchedthrough diode 405.

[0026] During a transmit mode, diode 405 generates a light output at oneof multiple power levels. In one application, the power output of diode405 is modulated to produce a time-varying output at power level P₀ andP₁ as shown in FIG. 1. To achieve this end, a bias current, I_(BIAS),can be switched through diode 405 to produce a baseline optical output,P₀. This puts the diode in a lasing or near-lasing mode. A modulationcurrent, I_(MOD) can then be selectively switched through diode 405 andan alternate path to transmit a stream of binary data to a targetreceiver disposed to receive the light output over fiber 430. When bothI_(BIAS) and I_(MOD) are switched to pass through diode 405, diode 405produces a light output at power level P₁. Thus, the light output ofdiode 405 can be varied between P₀ and P₁ by driving diode 405 withI_(BIAS) and selectively switching modulation current I_(MOD).

[0027] Current source 585 can be adjusted to provide a bias currentrange typically between 1 and 100 mA (milliamps). However, any suitablecurrent range can be used depending on the characteristics of the solidstate device that is to be biased and driven. Current sources 535, 585typically are less sensitive to temperature and can drive current to anaccuracy of 0.01% that varies by 50 ppm/deg C.

[0028] Digital potentiometer 575 defines the resolution of steps in agiven current range supported by current source 585. Consequently, biascontrol signal 514 can be digital information that selects a particularbias current selectively switched through diode 405. In one application,the bias current and modulation current are selected so that diode 405can be driven in its non-linear region.

[0029] Selective switching of the bias current, I_(BIAS), through diode405 for lasing can be achieved via switch Q572 and switch Q574. Althoughthese switches are shown as bipolar junction transistors, generally anysuitable type of switches for redirecting current or changing theimpedance of a circuit path can be used. Typically, electronic switchesare used for high speed switching.

[0030] Depending on components used in FIG. 3, diode 405 can be biasedin less than 1 nanosecond compared to 100 nanoseconds or more asachieved in the prior art. Accordingly, an extinction ratio, 10 log(P₁/P₀), of 7 to 30 dB is typically achieved.

[0031] A combination of Q572 and Q574 can comprise a differentialtransistor pair that is disposed in a single package. Gains of eachswitch can be controlled so that they are approximately equal, but thisis not necessary. As shown, the emitters of each switch are tied to acommon node of current source 585.

[0032] Differential driver 560 generates −BIAS and +BIAS signals thatdrive switch Q574 and switch Q572, respectively. During operation, abase of each transistor is driven with current to control through whichswitch or circuit path the bias current will pass. A gain of theswitches can vary depending on the application but transistors that havea gain, G, between 20 and 100 are typically used.

[0033] In one application, differential driver 560 generates an outputat voltage levels that do not cause corresponding switches to go intosaturation. For example, a selected switch such as Q572 can be activatedmore than Q574 so that almost all of the bias current flows through apath including diode 405 and R570. Conversely, switch Q574 can beactivated more than Q572 so that the bias current I_(BIAS) flows throughan alternate path such as through resistor R565. Faster switching can beachieved when the switches are not saturated, although the circuit willoperate at high speed even though the transistors are saturated.Resistors R530, R531, R580, and R581 can be chosen so that correspondingswitches do not go into saturation when driven to the ‘on’ state.

[0034] Current source 585 can be coupled to Vee as shown and ECL(Emitter Coupled Logic) voltage levels can be used to drive switchesQ572 and Q574. More specifically, differential driver 560 can drive−BIAS to −1.745 volts and +BIAS to −0.945 volts to switch I_(BIAS)through diode 405. Conversely, differential driver 560 can drive −BIASto −0.945 volts and +BIAS to −1.745 volts to switch I_(BIAS) throughalternate path R565 so that diode 405 is not biased. PECL (PositiveEmitter Coupled Logic), LVPECL (Low Voltage Positive Emitter CoupledLogic), or other suitable voltage levels can be used to bias theswitches.

[0035] One aspect of the present invention as shown in FIG. 3 involvesapproximating or substantially matching the impedance of circuit pathsso that a selected bias current can be maintained by current source 585even though the current is switched to travel between different paths.For example, resistor R565 and R570 can be independently chosen so thatcurrent source 585 can precisely and accurately drive a selected currentI_(BIAS) that is relatively stable or constant even though it isswitched between paths including Q572 and Q574.

[0036] Normally, resistors R570 and R565 are chosen in a range between 7and 50 ohms. In one application, R565 is 15 ohms and R570 is 10 ohms.However, any suitable range of resistors can be used depending on theapplication.

[0037] To reduce noise during transition periods of biasing, a snubbercircuit or higher resistor value for R570 greater than 20 ohms can beused for R570 and R565.

[0038] It should be noted that functionality provided by switches Q572and Q574 can be incorporated into current source 585. In this instance,current source 585 can be driven with a digital ‘enable’ signal thatcauses bias current I_(BIAS) to pass through diode 405 or an alternatepath.

[0039] Similar to circuit 595, circuit 545 can be used to control themodulation current, I_(MOD), through diode 405.

[0040] More specifically, data current control information 512 drivesdigital potentiometer 525 to adjust current source 535 so that itproduces a selected modulation current. The modulation current, I_(MOD),is then selectively switched through switch Q522 or switch Q524depending on the state of signal −DATA and +DATA from correspondingdifferential driver 510.

[0041] The input signal to driver 510 can be binary data such assequences of logic ones and zeros. A time varying sequence of digitaldata can be modulated onto a light output of diode 405 by imparting avoltage at the input of driver 510 to a high or low logic state at theappropriate time. Typically, bias current I_(BIAS) is set up andswitched through diode 405 prior to the switching of the modulationcurrent, I_(MOD), through diode 405 to produce a modulated light outputonto fiber 430 or other medium. As previously discussed, selectiveswitching of I_(MOD) results in a light output from diode 405 between P₀and P₁.

[0042] Resistors R515 and R520 are typically chosen from a range between7 and 50 ohms. In one application, R515 is 15 ohms and R520 is 10 ohms.However, any suitable range of resistors can be used depending on theapplication.

[0043] Resistors R515, R520, R565, and R570 can be greater than 20 ohmsso that the output of diode 405 makes a smooth transition when switchedbetween levels P₀ and P₁.

[0044] Resistors R520 and R570 can be combined as resistor R950 toreduce circuit components. Otherwise, R950 can be a jumper or zero ohmresistor while R520 and R570 are chosen to be values as previouslydiscussed.

[0045]FIG. 4 is a timing diagram illustrating a method of transmittingdata information according to certain principles of the presentinvention. As shown, a frame of data or data packet is transmitted as atime-varying light output signal from diode 405.

[0046] Prior to data transmissions at time t₂, current source 535 iscontrolled or adjusted to produce a selected modulation current I_(MOD)that is initially switched to pass through the circuit path includingR515 and Q522. Likewise, current source 585 is controlled to produce abias current, I_(BIAS), that is initially switched through the circuitpath including Q574 and R565. In other words, switches Q574 and Q522 areinitially switched to the “ON” state while switches Q572 and Q524 areswitched “OFF” while the current is set to an appropriate value.

[0047] Current sources 535, 585 can be adjusted between frames orpre-set to a selected value in anticipation of transmitting future data.

[0048] At time t₁, the bias current generated by current source 585 isthen switched through Q572, R570 and diode 405. Typically, diode 405 canbe biased on the order of nanoseconds or picoseconds with a high speedswitch. In one application, diode 405 can be biased in less than asingle bit time and has a rise time of 150 to 200 picoseconds. This canbe achieved because switches are used to adjust how much current passesthrough the diode rather than adjusting current source 585 between 0.0mA and I_(BIAS). Typically, current source 585 is a precision devicethat can accurately drive a selected current. However, the complexcircuitry supporting such precision does not lend itself to high speedcurrent adjustments that would be necessary to rapidly turn diode 405 onand off for transmitting.

[0049] Following set up time, t_(setup), binary data information ismodulated onto light output of diode 405 by switching the modulationcurrent between paths corresponding with switches Q522 and Q524. Morespecifically, the modulation current produced by current source 535 isswitched to pass through switch Q524 and diode 405 between time t₂ andt₃. Thereafter, the modulation current is switched back through switchQ522 between time t₃ and t₄. Accordingly, this technique of switchingI_(MOD) is repeated over the duration of a time frame to modulate binarydata onto the light output of diode 405.

[0050] As shown between t₆ and t₇, the bias current can be switchedagain through Q574 instead of Q572 so that diode 405 is no longer biasedfor transmitting data. Note also that the modulation current I_(MOD) isalso switched through the circuit path including switch Q522 so thatlittle or no current flows through diode 405 when it is not biased. Thefast turn-on and turn-off time of switches Q572 and Q574 to bias diode405 renders it possible for multiple burst mode laser systems aspreviously described to transmit without interfering with each other ina TDM (Time Division Multiplexed) communication system.

[0051] A time frame can include transmitting 55 bytes of information inwhich 53 bytes are encoded data and 2 bytes are framing information. Inone application, a time frame is 700 nanoseconds in duration and eachbit is on the order of 1.59 nanoseconds. Accordingly, a network systemcan support a bandwidth of 622 MBS, but these specifications can varydepending on the application.

[0052]FIG. 5 is a diagram of an optical access system according tocertain principles of the present invention. As shown, multiple remoteterminal devices 710-1, 710-2, . . . 710-n communicate over a fibernetwork such as a Passive Optical Network (PON) to transmit and receivedata information from central terminal 704. Each remote terminal device710 can include both a transmitter and receiver for transmitting andreceiving optical signals to and from central terminal 704.

[0053] In one application, each of the remote terminal devices 710receives an optical signal that is continuously, periodically orintermittently transmitted from central terminal 704. To differentiatedata flows from the central terminal 704 to remote terminals 710,central terminal 704 can generate downstream optical signals based onTDM (Time Division Multiplexed) standards or another suitable protocol.Consequently, central terminal 704 can simultaneously maintainindividual communication links with each of the remote terminals 710.

[0054] In a reverse direction, the remote terminals 710 can communicatewith central terminal 704 using a communication protocol such as TDM(Time Division Multiplexed) techniques. In this instance, the remoteterminals 710 can be dynamically assigned one or multiple time frames inwhich to transmit bursts of data information to the central terminal704. Time frames can be assigned for use on an as-needed basis bycentral terminal 704. Based on the coordination of multiple burst modelaser systems in each of the remote terminals 710, data information canbe efficiently communicated over fiber network 700 to central terminal704. More specifically, the bandwidth of information transmitted on afiber network can be optimized so that minimal time is wasted setting upeach of the laser diodes for transmitting data information.

[0055] It should be noted that other suitable protocols or standards fortransmitting data information from a remote terminal 710 to centralterminal 704 can be used in lieu of TDM techniques.

[0056]FIG. 6 is a graph illustrating attenuation of correspondingoptical signals transmitted from each of multiple burst mode lasersystems according to certain principles of the present invention. Asshown, an optical signal as transmitted from each remote terminal device710 can be attenuated by combiners, and other non-ideal componentsdisposed in optical system 700.

[0057] One aspect of the present invention as discussed is directedtowards reducing the time it takes to bias a diode laser so that it canbe used almost immediately to transmit data to a target withoutinterfering with other devices sharing optical system 700. To achievethis end, laser diodes in each of the remote terminal devices 710 can bealmost completely shut off when data information is not actively beingtransmitted. Consequently, there is little if any offset light receivedat central terminal 704 from diodes that are not biased to transmitdata. When the individual burst mode laser devices are specificallymodulated and biased during a time frame in this way, it is a simplertask to receive and demodulate a corresponding signal at centralterminal 704. For example, it is easier to retrieve a modulated signalwith a receiver at central terminal 704 when unnecessary light generatedby each of the non-transmitting laser diodes is reduced or minimized.

[0058]FIG. 7 is a block diagram of a network system including multipleburst mode laser devices according to certain principles of the presentinvention. As shown, central terminal 704 can be used to tap intonetwork 920 such as a SONET ring network. Thus, digital processordevices 950-1, 950-2 . . . 950-m can be coupled to remote terminals 710for processing received data and generating data to be transmitted overpassive optical network 930.

[0059] In one application, remote device 943 coupled to network 940 cancommunicate through digital processor device 950-2 such as a server andnetwork 930 to other devices in communication with network 920. The FSAN(Fiber Service Access Network) protocol can be used to communicate dataon network 930.

[0060] While this invention has been particularly shown and describedwith references to preferred embodiments thereof, it will be understoodby those skilled in the art that various changes in form and details maybe made therein without departing from the scope of the inventionencompassed by the appended claims.

What is claimed is:
 1. A method of supporting communication comprising:setting a current source to provide a first current; switching the firstcurrent through a diode or an alternate path to selectively bias thediode or shunt the first current; and modulating a light energy outputtransmitted from the diode by selectively switching a second currentthrough the diode while the first current is switched to bias the diode,wherein the light energy output of the diode is proportional to a totalcurrent through the diode.
 2. A method as in claim 1 further comprising:coupling the light energy output of the diode to a fiber andtransmitting information over an optical network via time divisionmultiplexing.
 3. A method as in claim 1 further comprising: disposing aresistor in series with the diode, through which the first and secondcurrent flow when the diode is in a biased mode.
 4. A method as in claim3, wherein the resistor is greater than 20 ohms.
 5. A method as in claim1 further comprising: coupling the light energy output of the diode to afiber of a passive optical network.
 6. A method as in claim 5, whereinthe passive optical network is coupled to a SONET ring network.
 7. Amethod as in claim 1 further comprising: disposing a first switch inseries with the diode and a second switch along the alternate path; andcontrolling the first switch to have a lower impedance than the secondswitch to selectively switch the first current to bias the diode.
 8. Amethod as in claim 1 further comprising: disposing a first switch inseries with the diode and a second switch along the alternate path; andcontrolling the second switch to have a lower impedance than the firstswitch to steer the first current through the alternate path.
 9. Amethod as in claim 7 further comprising: driving the first and secondswitches with a differential signal to selectively bias the diode.
 10. Amethod as in claim 9, wherein the differential signal is based on ECL(Emitter Coupled Logic) voltage levels and the switches are bipolartransistors.
 11. A method as in claim 1, wherein an extinction ratio ofthe light energy output of the diode while biased is greater than 15 dB.12. A method as in claim 1 further comprising: in between transmittingdata packets, adjusting the first or second current.
 13. A method as inclaim 1, wherein the alternate path has an impedance similar to a pathincluding the diode.
 14. A method as in claim 1 further comprising:switching the first and second currents through the diode based onrespective outputs of a first and second differential driver.
 15. Amethod as in claim 14, wherein an output of the first differentialdriver is switched to either of two states to selectively bias the diodeand the second differential driver is switched between either of twostates to modulate the diode.
 16. An apparatus for supportingcommunication comprising: a first current source that generates a firstcurrent; first and second switches for respectively switching the firstcurrent through a diode to bias the diode or shunt the first currentthrough an alternate path; and a second current source that generates asecond current that is selectively switched through the diode tomodulate a light energy output that is transmitted from the diode whilethe first current is switched to bias the diode, wherein the lightenergy output of the diode is proportional to a total current throughthe diode.
 17. An apparatus as in claim 16, wherein the light energyoutput of the diode is coupled to a fiber to transmit information overan optical network via time division multiplexing.
 18. An apparatus asin claim 16 further comprising: a resistor in series with the diode, thefirst current also flowing through the resistor when the diode isbiased.
 19. An apparatus as in claim 16, wherein the resistor has aresistance greater than 20 ohms.
 20. An apparatus as in claim 16 furthercomprising: a fiber of a passive optical network to which the lightenergy output of the diode is coupled.
 21. An apparatus as in claim 20,wherein the passive optical network is coupled to a SONET ring network.22. An apparatus as in claim 16, wherein the first switch is disposed inseries with the diode and a second switch is disposed along thealternate path.
 23. An apparatus as in claim 22 further comprising: afirst signal that, depending on its voltage level, selectively activatesthe first switch to have a lower impedance than the second switch todirect the first current through the diode; and a second signal that,depending on its voltage level, selectively activates the second switchto have a lower impedance than the first switch to selectively switchthe first current through the alternate path and not through the diode.24. An apparatus as in claim 23 further comprising: a driver thatgenerates a differential output comprising the first and second signalsthat are used to selectively bias the diode.
 25. An apparatus as inclaim 24, wherein the differential output is based on ECL (EmitterCoupled Logic) voltage levels and the first and second switches arebipolar transistors.
 26. An apparatus as in claim 16, wherein anextinction ratio of the light energy output of the diode while biased isgreater than 15 dB.
 27. An apparatus as in claim 16, wherein the firstor second current is adjusted in between transmitting data packets. 28.An apparatus as in claim 16, wherein the alternate path has an impedancesimilar to a path including the diode.
 29. An apparatus as in claim 16,wherein the first and second currents are respectively switched throughthe diode based on an output of a first and second differentialamplifier.
 30. An apparatus as in claim 29, wherein an output of thefirst differential amplifier is switched to either of two states toselectively bias the diode and the second differential amplifier isswitched between either of two states to modulate the diode.
 31. A burstmode laser driver comprising: a modulation differential transistor paircoupled to a laser diode, the modulation differential transistor paircontrolling a modulation current through the laser diode; and a biasdifferential transistor pair coupled to the laser diode, the biasdifferential transistor pair controlling a bias current through thelaser diode and switching the bias current on or off in less than asingle bit time of modulated data.
 32. A method for high speed switchinga diode comprising: producing a first differential output comprisingcomplementary first and second switch control signals; and setting thefirst differential output to either of two states to selectively biasthe diode for high speed switching.
 33. A method as in claim 32 furthercomprising: producing a second differential output comprisingcomplementary third and fourth switch control signals; and setting thesecond differential output to either of two states to selectivelymodulate the diode for transmitting binary information.
 34. A method asin claim 32, wherein the first and second control signals drive switchesthat are used to selectively switch a bias current through the diode oran alternate path.
 35. A method as in claim 33, wherein the third andfourth control signals drive switches that are used to selectivelyswitch a modulation current through the diode or an alternate path. 36.A high speed switching driver comprising: a first circuit device thatproduces a first differential output comprising complementary first andsecond signals, the first differential output being set to either of twostates; and a diode that is selectively biased depending on the firstdifferential output of the first circuit device.
 37. A high speedswitching driver as in claim 36 further comprising: a second circuitdevice that produces a second differential output comprisingcomplementary third and fourth signals, the second differential outputbeing set to either of two states; and a diode that is selectivelymodulated depending on the second differential output of the secondcircuit device.
 38. A method as in claim 36, wherein the first andsecond control signals drive switches that are used to selectivelyswitch a bias current through the diode or an alternate path.
 39. Amethod as in claim 37, wherein the third and fourth control signalsdrive switches that are used to selectively switch a modulation currentthrough the diode or an alternate path.